Microporous membrane lithium ion secondary battery and method of producing the microporous membrane

ABSTRACT

A method suppresses membrane thickness variation and air resistance variation after a compression at 60° C. or 80° C. Stretching is performed at least twice in at least different axial directions before the extraction of the solvent, and at the same time, at least one of (i) and (ii) is satisfied. (i) The step (c) is a first stretching step of stretching the sheet-shaped product at least once in a sheet transport direction (MD direction) and at least once in a sheet width direction (TD direction) individually, and the MD stretching magnification and the TD stretching magnification in the step (c) satisfy (TD stretching magnification≥MD stretching magnification−2). (ii) The stretching temperature (T1) of a first axial stretching performed firstly in the step (c) and the maximal stretching temperature (T2) of a second stretching performed after the first axial stretching satisfy (T1−T2≥0).

TECHNICAL FIELD

This disclosure relates to a microporous membrane, a lithium ionsecondary battery and a method of producing the microporous membrane.

BACKGROUND

Thermoplastic resin microporous membranes are widely used as separationmembranes, selectively permeable membranes, isolation membranes, and thelike of materials. Specific application examples of the microporousmembrane include separators for batteries used in lithium ion secondarybatteries, nickel-hydrogen batteries, nickel-cadmium batteries, andpolymer batteries, separators for electric double-layer capacitors,various filters such as reverse osmosis filtration membranes,ultrafiltration membranes and microfiltration membranes, breathablewaterproof clothing, medical materials, supports for fuel cells and thelike.

Especially as separators for lithium ion secondary batteries,polyethylene microporous membranes are broadly used. In addition totheir characteristics including the safety of the batteries andexcellent mechanical strength contributing greatly to the productivity,polyethylene microporous membranes also have ion permeability by anelectrolyte passing through micro pores while maintaining the electricalinsulation property, and also are provided with a pore-blocking functionto regulate the excessive temperature rise by blocking the penetrationof ions automatically at about 120 to 150° C. in an abnormal reactionoutside or inside of the battery.

Lithium ion batteries have expanded their use fields to large-sized andlarge capacity applications such as power tools, automobiles, bicyclestorage batteries, and large electrical storage equipment in addition toso-called small-sized consumer applications including conventionalmobile phones and batteries for PCs. Electrodes providing a highcapacity as a battery structure have been increasingly applied to meetthese demands. The electrode materials used for these electrodes have acharacteristic that expansion and shrinkage of the volume accompanied bythe charge and discharge is larger than those of conventionalelectrodes, and the separators have been required to show small changein the performance even when the electrodes are in expansion andshrinkage.

Separators have been developed to meet these demands (JP2012-038655 A,JP2007-262203 A, WO2008-093572, Japanese National-Phase Publication2010-540692 and WO2006-106783).

JP2012-038655 A discloses, as a means of improving the durability of abattery, a microporous membrane film made of polypropylene andpolyphenylene ether, and the microporous film shows constant values ofstress and film thickness recovery rate when a flat indenter having adiameter of 50 μm is pushed 3 μm deep into the film.

In JP2007-262203 A, as a method of improving the compression resistance,a polyolefin microporous membrane in which small protrusions of the sizesmaller than 25% of the membrane thickness are introduced on themembrane surface has been developed. The stress and membrane thicknessrecovery rate are maintained within a certain range after themicroporous membrane cut out in a square of 50 mm was compressed at 55°C. for 5 seconds to the 80% thickness of the initial membrane thickness.

WO2008-093572 discloses a polyolefin microporous membrane, wherein themaximal pore size, the elastic modulus in the longitudinal direction,the ratio of the elastic modulus in the longitudinal direction to theelastic modulus in the width direction are defined, and a 9-μmmicroporous membrane has excellent compression resistance in Example 16.A polyolefin microporous membrane having “good strain absorptivity” andmaintaining good ion permeability after the compression is shown.

Japanese National-Phase Publication 2010-540692 discloses a microporousmembrane in which the injection of electrolyte is improved by regulatingthe polyethylene composition and, in Examples 1 to 3, polyolefinmicroporous membranes show a small membrane thickness variation afterthe compression and have an air resistance of 500 seconds/100 ml orless. Japanese National-Phase Publication 2010-540692 has acharacteristic of a pore size distribution curve having at least twopeaks.

WO2006-106783 discloses a microporous membrane having a membranethickness variation of 15% or less after heating at 2.2 MPa and 90° C.for 5 minutes and an air resistance of 700 seconds/100 ml/20 μm or lessafter a heat compression, and such physical properties are achieved byre-stretching the membrane at least in the monoaxial direction after theremoval of a membrane-forming solvent.

Along with the increase in the size and capacity of lithium ionbatteries, electrodes suitable for higher capacity as a battery materialhave been increasingly applied. As a method of increasing the capacity,at least one of the following has been studied: 1) adopting a newelectrode material and 2) increasing the electrode material density inthe battery. The new electrode material used in the method of 1) has acharacteristic that the expansion and shrinkage accompanied by thecharge and discharge is larger than those in conventional electrodes,and a problem of the deterioration of the battery performance withrepeated charges and discharges has been revealed. As a measure for thatproblem, the separator has been required to show a small change in theperformance even when the electrodes are in expansion and shrinkage.Moreover, to develop the method 2), thinning of the separator has beendeveloped. At the same time, as the size of batteries are increasing, itis also necessary to consider that the temperature inside the batteryduring the charge and discharge gets higher than before.

JP2012-038655 A studies the compression property of a micro region andproposes a separator with a compression resistance. However, since thecompression property of the micro region is studied, stress is relaxedaround the region during the compression, and it was difficult toaccurately simulate the compressed state inside the battery which issubject to stress in a wider area. In JP2012-038655 A, polyphenyleneether (PPE) in polypropylene is dispersed and thus the compressionresistance is improved by voids from PPE, but there still remained aproblem in the strength. JP 2012-161936 A, Example 1, 2.5 N/30 μmdiscloses, as related art, an improved microporous membrane, but itstill requires a further improvement in strength.

JP2007-262203 A considers a compression behavior of a relatively largearea, but the results were from the evaluation of compression resistancefor a short time and, hence did not reflect a condition which can occurin a practical battery.

In WO2008-093572, as a means of improving the compressibility, amembrane which is easily deformed is considered. Despite the compressionprocess for a relatively short time, the membrane thickness changed by 4μm or more, and even the smallest change in the air resistance showed adeterioration by 2.6 times compared to before the compression.

By studying raw material compositions, stretching methods after asolvent removal, a microporous membrane and a separator which areexcellent in compression resistance have been developed to improve thecompression characteristics of a separator. As a countermeasure to theexpansion and shrinkage of an electrode, the following separators havebeen developed: a) a separator which withstands stress applied to theseparator and which shows a small variation in the membrane thickness,and b) a separator in which the membrane thickness slightly decreases inresponse to the pressure from the electrode, but the variation in ionpermeability is small.

Under the model of a), Japanese National-Phase Publication 2010-540692achieves the improvement of the injection of electrolyte and compressionresistance by adjusting the polyethylene composition and regulating thepore size distribution, but the improvement of the performance was stillrequired.

Under the model of b), WO2006-106783 proposes a polyolefin microporousmembrane having improved compression resistance by re-stretching afterthe extraction of a plasticizer. The microporous membrane caused amembrane thickness variation of more than 15% after a compression at 2.2MPa and 90° C. for 5 minutes, but showed an improved compressionresistance. However, the suppression quality of the deterioration of theair resistance needed to be improved, and the strength was alsoinsufficient.

In recent years, as the variation in ion permeability is required to besmall, the mainstream has been the development under the model of b)which shows a smaller degree of deterioration of air resistance thanunder the model of a). In addition, those studies have been mainly onseparators which have a membrane thickness of about 20 μm and thus haveroom for reduction in the membrane thickness.

On the other hand, with the recent increase in capacity of batteries,thinner separators have been increasingly applied to increase theelectrode density as much as possible, and the membranes have beenthinned from the conventional thickness of about 20 μm to 15 μm or less.A large variation in the membrane thickness under the compressionconditions may cause insulation failure or the possibility of a thinnerseparator after the compression by the electrode during the charge anddischarge of the battery. Therefore, it is not possible to maximize theelectrode density inside the battery at the time of the manufacturing ofthe battery. The prior model based on the premise that the membranethickness decreases to some extent (the model of the above b)) cannotoffer a sufficient solution, and the deterioration of ion permeabilityaccompanying the charge and discharge has become a major subject fordevelopment.

WO2008-093572 discloses a separator having excellent compressionresistance at 15 μm or less, but the separator disclosed inWO2008-093572 has small strength, and there is still room forimprovement in the battery productivity. Moreover, the membranethickness of the separator greatly decreases, resulting in voids in thebattery after the charge and discharge. Thus, the necessity to maximizethe electrode in the battery was not satisfied.

In addition to those necessities, the safety which has been required forseparators so far is still important as before. It is required to safelystop and maintain the battery function when the battery is heatedabnormally, but this is a more difficult task for a thin membraneseparator in which the amount of resin between electrode plates isgreatly reduced.

Among the physical properties possessed by the microporous membrane as aseparator, the heat shrinkage rate, shutdown, and meltdown property, forexample, greatly contribute to the safety of the battery. When thebattery abnormally generates heat, a short circuit may occur at the endof the electrode due to heat shrinkage of the separator. Thus,management of the heat shrinkage rate of the separator is important.Furthermore, in the shutdown function that stops the battery functionsafely by closing pores of the separator in an abnormal heat generationof the battery and blocking the ion conductivity, the shutdowntemperature (SDT) is required to be lowered, and then the meltdowntemperature (MDT) at which the ion conductivity returns again isrequired to be increased after the shutdown. Thus, the battery functioncan be safely stopped when heat is generated abnormally. Therefore, itis important not only to lower the temperature of the SDT but also towiden the temperature difference between the SDT and the MDT for theassurance of the battery safety.

We provide:

(1) A microporous membrane, wherein the average membrane thickness is 15μm or less, the air resistance is 400 seconds or less, and the airresistance variation (60° C.) after a pressurization treatment under theconditions of 60° C., 4 MPa, and 10 minutes is 148% or less: whereinAir resistance variation(60° C.)=air resistance after the pressurizationtreatment at 60° C./air resistance prior to the pressurizationtreatment×100.

(2) The microporous membrane according to (1), wherein said airresistance variation is 145% or less.

(3) The microporous membrane according to (1) or (2), wherein the airresistance variation (80° C.) after a pressurization treatment under theconditions of 80° C., 4 MPa, and 10 minutes is 200% or less: whereinAir resistance variation(80° C.)=air resistance after the pressurizationtreatment at 80° C./air resistance prior to the pressurizationtreatment×100.

(4) The microporous membrane according to (3), wherein said airresistance variation (80° C.) is 190% or less.

(5) A microporous membrane, wherein the average membrane thickness is 15μm or less, the air resistance is 400 seconds or less, and the airresistance variation (80° C.) after a pressurization treatment under theconditions of 80° C., 4 MPa, and 10 minutes is 200% or less: whereinAir resistance variation(80° C.)=air resistance after the pressurizationtreatment at 80° C./air resistance prior to the pressurizationtreatment×100.

(6) The microporous membrane according to (5), wherein said airresistance variation is 190% or less.

(7) A microporous membrane, wherein the average membrane thickness is 15μm or less, and the variation ratio (80° C./60° C.) of the airresistance variation (80° C.) after a pressurization treatment under theconditions of 80° C., 4 MPa, and 10 minutes to the air resistancevariation (60° C.) after a pressurization treatment under the conditionsof 60° C., 4 MPa, and 10 minutes is 130% or less: whereinVariation ratio(80° C./60° C.)=(air resistance variation(80° C.)/airresistance variation(60° C.))×100,wherein,Air resistance variation(80° C.)=air resistance after the pressurizationtreatment at 80° C./air resistance prior to the pressurizationtreatment×100, andAir resistance variation(60° C.)=air resistance after the pressurizationtreatment at 60° C./air resistance prior to the pressurizationtreatment×100.

(8) The microporous membrane according to (7), wherein said variationratio (80° C./60° C.) is 122% or less.

(9) The microporous membrane according to any one of (1) to (8), whereinthe pin puncture strength corresponding to the membrane thickness of 12μm is 4000 mN or more, the heat shrinkage rate after an exposure at 105°C. for 8 hours is 5% or less, and the average tensile rupture elongationis 130% or less.

(10) The microporous membrane according to any one of (1) to (9),wherein the shutdown temperature is 140° C. or less, or the temperaturedifference between the shutdown temperature and the meltdown temperatureobtained by a temperature-increasing air permeability method is 10° C.or more.

(11) The microporous membrane according to any one of (1) to (10),wherein the average pore size is 0.1 μm or less.

(12) The microporous membrane according to any one of (1) to (11),comprising more than 2% of an ultra high molecular weight polyethylenecomponent having a weight average molecular weight of 1,000,000 or more,or 5% or more of a molecule component having a weight average molecularweight of more than 1,000,000.

(13) A microporous membrane comprising more than 2% of an ultra highmolecular weight polyethylene component having a weight averagemolecular weight of 1,000,000 or more, or 5% or more of a moleculecomponent having a weight average molecular weight of more than1,000,000, wherein the average membrane thickness is 15 μm or less, theair resistance is 400 seconds or less, the pin puncture strengthcorresponding to the membrane thickness of 12 μm is 4000 mN or more, theheat shrinkage rate after an exposure at 105° C. for 8 hours is 5% orless, the average tensile rupture elongation is 130% or less, and theaverage pore size is 0.1 μm or less.

(14) The microporous membrane according to (13), wherein the shutdowntemperature is 140° C. or less, or the temperature difference betweenthe shutdown temperature and the meltdown temperature obtained by atemperature-increasing air permeability method is 10° C. or more.

(15) The microporous membrane according to any one of (1) to (14),wherein the formation of a hybrid structure of a ladder-like structurein a submicron region and a three-dimensional network structure in amicron region is observed on at least one surface of the microporousmembrane.

(16) A lithium ion secondary battery, wherein the microporous membraneaccording to any one of (1) to (15) is used.

(17) A method of producing a polyolefin microporous membrane,comprising:

-   (a) an extrusion step of melt blending and extruding a resin    composition containing a polyolefin resin and a pore-forming    material,-   (b) a sheet forming step of sheet forming the extrudate obtained in    said step (a) into a sheet,-   (c) a first stretching step of stretching the sheet-shaped product    obtained in said step (b) at least twice in at least different axial    directions,-   (d) an extraction step of extracting the pore-forming material from    the stretched sheet obtained in said step (c), and-   (e) a second stretching step of stretching the sheet obtained in    said step (d) at least once in at least one axial direction,-   wherein at least one of the following (i) and (ii) is satisfied:-   (i) the step (c) is a first stretching step of stretching the    sheet-shaped product at least once in a sheet transport direction    (MD direction) and at least once in a sheet width direction (TD    direction), and the MD stretching magnification and the TD    stretching magnification in the step (c) satisfy the following    Equations (1-1) and (1-2):    TD stretching magnification≥MD stretching magnification−α  Equation    (1-1)    α=2.0  Equation (1-2),-   (ii) the stretching temperature (T1) of a first axial stretching    performed firstly in the step (c) and the maximal stretching    temperature (T2) of a second stretching performed after the first    axial stretching satisfy the following Equations (2-1) and (2-2),    T1−T2≥β  Equation (2-1)    β=0  Equation (2-2).

(18) The method of producing a microporous membrane according to (17),wherein said (i) and said (ii) are satisfied simultaneously.

Although our polyolefin microporous membrane is a thin membrane, theincrease ratio of the air resistance is low under heating conditions ata high temperature that can occur in high capacity and high outputbatteries, and the performance deterioration as a battery is small, andthus suitable as a separator of a lithium ion secondary battery. Inaddition, the polyolefin microporous membrane is excellent in pinpuncture strength, heat shrinkage rate and processability in spite ofits thin membrane.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an AFM photo showing a surface image obtained from amicroporous membrane.

FIG. 2 is an AFM photo showing a surface image obtained from amicroporous membrane.

FIG. 3 is an AFM photo showing a surface image obtained from amicroporous membrane produced by the conventional simultaneous biaxialstretching method.

FIG. 4 is an AFM photo showing a surface image obtained from amicroporous membrane produced by the conventional simultaneous biaxialstretching method.

FIG. 5 is an AFM photo showing a surface image obtained from amicroporous membrane produced by the conventional stepwise biaxialstretching method.

FIG. 6 is an AFM photo showing a surface image obtained from amicroporous membrane produced by the conventional stepwise biaxialstretching method.

DETAILED DESCRIPTION

To obtain a polyolefin microporous membrane is excellent in pin puncturestrength, heat shrinkage and processability while having a membranethickness of 15 μm or less, and also having a small performancedeterioration even under heating conditions at a high temperature, wefound that a polyolefin microporous membrane whose air resistance, whichis an index of ion permeability, does not greatly deteriorate even byhigh temperature pressurization can be obtained by regulating thestretching conditions under certain conditions and forming uniforminteraction of ultra high molecular weight components.

Raw Materials

Resin Types

Preferred examples of polyolefin resins forming the polyolefinmicroporous membrane are polyethylene and polypropylene. The polyolefinresin may be a single resin or a mixture of two or more differentpolyolefin resins, for example, a mixture of polyolefin resins selectedfrom polyethylene, polypropylene, polybutene, andpoly4-methyl-1-pentene. The polyolefin resin is not limited to ahomopolymer of a single type and may be a copolymer of differentolefins. Among such polyolefin resins, polyethylene is particularlypreferred from the viewpoint of its excellent pore-blocking performance.The melting point (softening point) of polyethylene is preferably 70 to150° C. from the viewpoint of the pore-blocking performance.

The polyolefin resin is explained below in detail, using polyethylene asan example. Examples of polyethylene include ultra high molecular weightpolyethylene, high density polyethylene, medium density polyethylene,low density polyethylene and the like. The polyethylene polymerizationcatalyst is not particularly limited, and a Ziegler-Natta catalyst, aPhillips catalyst, a metallocene catalyst, or the like can be used.These polyethylenes are not limited to homopolymers of ethylene but alsomay be copolymers containing a small amount of another α-olefin.Suitable examples of α-olefins other than ethylene include propylene,1-butene, 1-pentene, 1-hexene, 4-methyl-1-pentene, 1-octene,(meth)acrylic acid, esters of (meth)acrylic acid, styrene, and the like.The polyethylene may be a single substance, but preferably apolyethylene mixture containing two or more kinds of polyethylenes.

As the polyethylene mixture, a mixture of two kinds or more of ultrahigh molecular weight polyethylenes having different weight averagemolecular weights (Mw), of high density polyethylenes, of medium densitypolyethylenes, of low density polyethylenes, or a mixture of two kindsor more selected from ultra high molecular weight polyethylenes, highdensity polyethylenes, medium density polyethylenes and low densitypolyethylenes may be used. As the polyethylene mixture, a mixture ofultra high molecular weight polyethylene having a Mw of 5×10⁵ or moreand polyethylene having a Mw of 1×10⁴ or more and 5×10⁵ or less ispreferred. The content of the ultra high molecular weight polyethylenein the polyethylene mixture is preferably from 1 to 70% by weight fromthe viewpoint of the tensile strength. The content of the ultra highmolecular weight polyethylene in the polyethylene mixture is morepreferably 2 to 65% by weight, and further preferably 5 to 60% byweight. When the ultra high molecular weight polyethylene is present inthe polyethylene mixture in the proportion of 1% by weight or more, atleast one of the membrane thickness variation and the air resistancevariation during the compression can be suppressed, and the membranestrength is improved compared to the case of less than 1% by weight ofthe ultra high molecular weight polyethylene. When the content of theultra high molecular weight polyethylene present in the polyethylenemixture is 70% by weight or less, the resin extrusion productivityimproves.

The molecular weight distribution of polyethylene (the weight averagemolecular weight (Mw)/the number average molecular weight (Mn)) ispreferably 5 to 200 from the viewpoint of the mechanical strength.

A polyethylene mixture containing 2% or more of an ultra high molecularweight polyethylene component having a weight average molecular weightof 1,000,000 or more or a polyethylene mixture containing 5% or more ofa molecule component having a weight average molecular weight of1,000,000 or more is desirably used. A polyethylene mixture containing3% or more of an ultra high molecular weight polyethylene componenthaving a weight average molecular weight of 1,000,000 or more or apolyethylene mixture containing 6% or more of a molecule componenthaving a weight average molecular weight of 1,000,000 or more is moredesirably used. The polyethylene component having a weight averagemolecular weight of 1,000,000 or more is preferably contained becausethe average pore size of the separator can be controlled to 0.1 μm orless, thereby preventing the deterioration of the separator performanceunder high temperature and high pressure conditions and the generationof metallic lithium dendrites accompanying the charge and discharge inthe battery.

Solvent Type: Pore-Forming Material

The diluent is not particularly limited as long as it is a substancewhich can be mixed with the polyolefin resin or a substance which candissolve the polyolefin resin. In the melt blending state with thepolyolefin resin, a solvent which is miscible with polyolefin but existsin a solid state at room temperature may be mixed with the diluent.Examples of such solid diluents include stearyl alcohol, ceryl alcohol,paraffin wax and the like. Examples of liquid diluents includealiphatic, cycloaliphatic or aromatic hydrocarbons such as nonane,decane, decalin, paraxylene, undecane, dodecane, liquid paraffin and thelike, mineral oil fractions which have a corresponding (same orequivalent) boiling point to that of these aliphatic, cycloaliphatic oraromatic hydrocarbons, and phthalate esters which are liquid at roomtemperature such as dibutyl phthalate and dioctyl phthalate, vegetaloils such as soybean oil, castor oil, sunflower oil, cotton oil, andother fatty acid esters. A nonvolatile diluent such as liquid paraffinis more preferably used to obtain a gel-like sheet having a stablecontent of the liquid diluent. For example, the viscosity of the liquiddiluent is preferably 20 to 500 cSt at 40° C., more preferably 30 to 400cSt, and further preferably 50 to 350 cSt. When the viscosity of theliquid diluent is 20 cSt or more, the extrudate from the die is uniformand the blending is also easy. When the viscosity of the liquid diluentis 500 cSt or less, the removal of the solvent (diluent) is easy.

For the mixing ratio of the polyolefin resin and the diluent, when thetotal of the polyolefin resin and the diluent is considered as 100% bymass, 1 to 60% by mass of the polyolefin resin is preferred from theviewpoint of good sheet formability of the extrudate. The proportion ofthe polyolefin resin to the mixture of the polyolefin resin and thediluent is more preferably 10 to 55% by weight, and further preferably15 to 50% by weight. When the ratio of the polyolefin resin to themixture of the polyolefin resin and the diluent is 1% by weight or more,the die swell and neck-in at the die exit during the extrusion can beprevented, resulting in improved membrane formability of a gel-likesheet. On the other hand, the ratio of the polyolefin resin to themixture of the polyolefin resin and the diluent of 60% by weight or lesscan maintain a small differential pressure at the die portion, resultingin the stable production of a gel-like sheet. The uniform melt blendingstep of the polyolefin solution is not particularly limited and include,in addition to a calendar and various mixers, an extruder equipped witha screw and the like.

Production Method

A production method by the wet method includes, for example, a method inwhich a microporous membrane is obtained by heating and melt blendingpolyethylene (polyolefin resin) and a solvent for sheet forming,extruding and cooling the obtained resin solution from a die to form anunstretched gel-like sheet, stretching the obtained unstretched gel-likesheet at least in a monoaxial direction, removing the solvent for sheetforming and drying the resulting.

The polyethylene microporous membrane may be a single layer membrane orhave a layer structure composed of two or more layers having differentmolecular weights or different average pore sizes. In a layer structurecomposed of two or more layers, the molecular weight and the molecularweight distribution of at least one outermost layer of the polyethyleneresin preferably satisfy the above ranges.

A multi-layer polyethylene microporous membrane composed of two or morelayers can be produced by any of the following examples: a method inwhich each polyethylene (polyolefin resin) constituting the layer A andthe layer B is heated and melt blended with a solvent for sheet forming,and each of the obtained resin solutions is introduced from respectiveextruders into one die and combined by co-extrusion, a method in whichgel-like sheets constituting each layer are overlaid on each other andthermally fused, a method in which the gel-like sheets are individuallyfused thermally after stretching, a method in which gel-like sheets arethermally fused after the solvent is removed. The co-extrusion method ispreferred because adhesion strength between layers can be easilyobtained, and easy formation of communication holes between the layersallows the permeability to be kept high, and the productivity isexcellent.

Mixing, Blending

The preferred range of the temperature of the polyolefin solution in theextruder depends on the resin. For example, for the polyethylenecomposition, a temperature of 140 to 250° C. is preferred, and whenpolypropylene is contained, the preferred range is 190 to 270° C. Thetemperature of the polyolefin solution in the extruder is indirectlymeasured by installing a thermometer inside the extruder or at acylinder portion, and the heater temperature, rotation speed andextrudate amount of the cylinder portion are appropriately adjusted sothat the target temperature is achieved. The diluent may be added beforethe blending or added during the blending. In melt blending, anantioxidant is preferably added to prevent oxidation of the polyolefinresin.

Extrusion and Casting

The polyolefin solution melt blended in the extruder is cooled to form adiluent-containing resin composition. In this example, the extrusionfrom a die having a slit-like opening is preferred in order to prepare asheet-like resin composition, but a so-called inflation method in whicha resin composition is solidified by extrusion from a die for a blownfilm having a circular opening can be also applied. The extrusiontemperature is preferably 140 to 250° C., more preferably 160 to 240°C., and further preferably 180 to 230° C. The extrusion temperature of140° C. or more can prevent the excessive increase in pressure at thedie portion, and the extrusion temperature of 250° C. or less canprevent the deterioration of materials. The extrusion rate (the sheetmembrane-forming rate) is preferably 0.2 to 15 m/min. A gel-like sheetis formed by cooling the polyolefin resin solution extruded in a sheetform. Cooling methods include a method of bringing the polyolefin resinsolution into contact with a coolant such as cold air, cooling water orthe like, a method of bringing the polyolefin resin solution intocontact with a cooling roll, or the like, and a method of bringing thepolyolefin resin solution into contact with a roll cooled with a coolantis preferred. For example, an unstretched gel-like sheet can be formedby bringing the polyethylene resin solution extruded in a sheet forminto contact with a rotary cooling roll having a surface temperature setat 20° C. to 40° C. with a coolant. The extruded polyethylene resinsolution is preferably cooled to 25° C. or lower. In this case, thecooling rate is preferably 50° C./min or more. Cooling methods as abovecan lead to a microphase separation of the polyolefin phase from thesolvent. This way, the unstretched gel-like sheet is likely to have adense structure, and the excessive increase of crystallinity can besuppressed, and thus, the unstretched gel-like sheet obtains a structuresuitable for stretching. As a method of cooling, to improve sheetcooling efficiency and sheet planarity, a method of cooling thepolyolefin resin solution by placing two or more types of rolls close toeach other and holding the resin solution extruded onto one roll withone or more rolls may be applied. Further, to form a gel-like sheet inthe high-speed membrane production, a chamber for attaching the sheetwith the roll may be used. The membrane thickness can be controlled byadjusting each extrusion amount of the polyolefin solution. As theextrusion method, for example, the methods disclosed in JPH06-104736 Band JP3347835 B can be used.

Stretching

A desired stretching method in the first stretching step is stretchingin two or more stages with the diluent contained. The stretching methodin each stage is not particularly limited. Preferred examples alsoinclude performing simultaneous biaxial stretching after monoaxialstretching, and performing monoaxial stretching after simultaneousbiaxial stretching. Considering the productivity and the investmentcost, an example of another monoaxial stretching after monoaxialstretching is also preferred. As for the stretching directions of thesheet transport direction (MD) and the sheet width direction (TD), forexample, the TD stretching may be performed after the MD stretching, orthe MD stretching may be performed after the TD stretching. Afterheating, a gel-like sheet can be stretched by a tenter method, a rollingmethod, a flatting method, or a combination thereof.

As an example, stepwise biaxial stretching in which roll-stretching isperformed in the MD direction and then stretching by a tenter method iscarried out in the TD direction is explained.

The stretching magnification before the extraction of the diluent variesdepending on the thickness of the gel-like sheet, but the MD stretching(MDO) by 2 times to 12 times is preferred. The MD stretchingmagnification before the extraction of the solvent is more preferably 3times to 12 times, further preferably 5 times or more and 11 times orless, and still further preferably 5.0 times or more and 11.0 times orless. The MD stretching by 2 times or more before the extraction of thesolvent allows for a uniform stretching. Therefore, during the TDstretching following the MD stretching, a formation of an unevenstructure in the MD direction can be avoided. The MD stretching by 5times or more before the extraction of the solvent results in a uniformmembrane thickness distribution in the MD direction and is preferred tocontrol the membrane quality (wrinkles, sagging) which will be importantin the post-processing. Further, the MD stretching can be performed intwo or more stages. In the MD stretching, the region where the MDstretching is applied is formed by a preheating part, a stretching part,and a thermal fixation part, and the temperature of the gel-like sheet(or the film being stretched) is controlled by heating and cooling withrolls in the above region. In the stretching part, a peripheral speeddifference between the rolls and a stretching section divided into aplurality of stages can be used to carry out the stretching. That is, inthe stretching part, the peripheral speed of the downstream side(winding side) roll adjacent to the roll on the most upstream side (thedie side) is increased, and the peripheral speed difference betweenthese two rolls is used to stretch the gel-like sheet. In this manner,the MD stretching in two stages or more (multistage) is performed asrolls with a peripheral speed higher than that of the upstream rolls arearranged sequentially on the downstream stage side. Specifically, whentwo pairs of rolls having different peripheral speeds from each other(two pairs of rolls which are set such that the peripheral speed of thedownstream side roll is faster than that of the upstream side roll) arearranged in the stretching part, the MD stretching is carried out in twostages, and when three pairs of the rolls are arranged in the stretchingpart, the MD stretching is carried out in three stages. Among thesepairs of rolls, the roll on the downstream side in any pair and the rollon the upstream side in a pair of rolls which is adjacent to and on thedownstream side of the above pair may be made common. For example, atwo-stage stretching section may be formed by three rolls.

The stretching magnification in each pair of rolls can be either equalor different. The stretching magnification in each stage is morepreferably different in a way that the stretching magnificationincreases on the more downstream side, thereby preventing further theincrease in the air resistance during the heat compression. Although thereason is uncertain, at an equal magnification, stretching is carriedout at a relatively high magnification at the initial stage ofstretching. When the stretching magnification is increased at differentmagnifications, it is presumed that the micro structure formed by the MDstretching tends to be uniform and the compression resistance isimproved.

The stretching in the TD direction after the MD stretching is preferablyby 2 to 12 times, more preferably 3 times to 12 times, and furtherpreferably 5 times to 10 times to improve the strength in the sheetwidth direction and the productivity. To form a uniform membranestructure in the TD direction (in order to form a uniform formation ofholes), the stretching magnification in the TD direction is desirably 2times or more. To obtain more uniform physical properties (airresistance, strength (pin puncture, tensile), heat shrinkage rate) inthe TD direction, the stretching magnification in the TD direction of 5times or more is more desired. The stretching magnification in the TDdirection of 12 times or less can prevent the variation of physicalproperties due to a high stretching magnification. The stretchingmagnification in the TD direction is further preferably 10 times or lessfrom the viewpoint of the production stability (to obtain uniformphysical properties in the TD direction while stabilizing theproductivity).

The total area magnification in the MD stretching and the TD stretchingbefore the extraction of the solvent is preferably 25 times or more,more preferably 30 times or more, and the most preferably 40 times ormore. The area magnification of the stretching before the extraction ofthe solvent is preferably 25 times or more to improve the strength. Theaverage tensile elongation is determined by the square root of theproduct of the elongation in the MD direction and the elongation in theTD direction, and represented by the formula, ((the elongation in the MDdirection)×(the elongation in the TD direction))^(0.5). This averagetensile elongation is preferably 130% or less. The stretching areamagnification before the extraction of the solvent is preferably set at40 times or more to obtain the average tensile elongation of 130% orless. On the other hand, the area magnification of the stretching beforethe extraction of the solvent is preferably 200 times or less, morepreferably 180 times or less, and the most preferably 150 times or less.When the stretching before the extraction of the solvent is carried outat the area magnification of 200 times or less, the stability isobtained during the membrane formation and is preferred from theviewpoint of the production of the microporous membrane.

The TD stretching magnification and the MD stretching magnificationbefore the extraction of the solvent preferably satisfy Equation (1-1).TD stretching magnification≥MD stretching magnification−α  (1-1)The α in Equation (1) represents the degree of orientation in theunstretched sheet as described later, and is a value derivedempirically.

By satisfying such a relationship, a microporous membrane excellent inhigh temperature compression characteristics (high temperaturecompression resistance) can be obtained. The reason can be assumed asfollows, although it is not clear. In our technique, since the structureis likely to be limited in the MD direction at the stage of theformation of the unstretched sheet, the stretching magnification in thesubsequent MD stretching and the structure (orientation) of theunstretched sheet are combined to obtain a structure oriented in the MDdirection. After the MD stretching, the structure in TD directionperpendicular to the MD direction is developed further by the TDstretching of a certain magnification or more. Thus, a more uniformmembrane is easily obtained. The degree of orientation resulting fromthe orientation during the formation of an unstretched sheet isconsidered as α. In consideration of this, when the ratio of the MDstretching magnification to the TD magnification is adjusted to behigher in the TD direction than in the MD direction, it is believed thata membrane in which fibrils are developed uniformly in the surfacedirection can be obtained. In summary, when the TD stretchingmagnification and the MD stretching magnification satisfy Equation(1-1), the pore-forming of the fibrils is likely to occur in both in theMD direction and in the TD direction. When the TD stretching isperformed after the MD stretching, a higher stretching magnification inthe TD direction during the pore-forming of the fibrils can facilitatethe interaction between the fibrils. The development of the interactionof the fibrils can form a structure whose physical properties at thetime of the compression are not likely to change. Therefore, the TDstretching magnification and the MD stretching magnification need to beregulated. The α is preferably 2.0, more preferably 1.5, furtherpreferably 1.0, still further preferably 0.5, and the most preferably0.0.

The stretching temperature in both of the MD stretching (MDO) and the TDstretching (TDO) is preferably the melting point of the polyolefin resinor less, and more preferably in the range of (the crystal dispersiontemperature Tcd of the polyolefin resin) to (the melting point of thepolyolefin resin −5° C.), and further preferably (the crystal dispersiontemperature Tcd of the polyolefin resin +5° C.) to (the melting point ofthe polyolefin resin −5° C.). For example, the stretching temperature inthe polyethylene resin is about 90 to 130° C., and more preferably 100to 127° C. The stretching temperature of the polyethylene resin isfurther preferably 105 to 125° C. When the stretching temperature of thepolyethylene resin is higher than or equal to the crystal dispersiontemperature of the polyolefin resin, the occurrence of micro cracks atthe time of stretching can be suppressed, thereby preventing a biggerpore size (particularly the maximal pore size, BP) in the end. As aresult, the permeation of ions becomes uniform, and Li dendrites areless likely to occur. Thus, a good battery performance is maintained. Inaddition, when the stretching temperature of the polyethylene resin isless than or equal to the melting point of the polyolefin resin, auniform stretching occurs, which prevents wrinkles and sagging and thusassures the productivity of the separator.

Considering that stretching is performed in the MD direction by arolling method after the stretching in the TD direction by a tentermethod, a relationship of each stretching temperature in performing thestretching in at least two or more stages is described in detail.

In the film production, the stretching is generally performed in multistages. Even the monoaxial stretching is performed in multi stages toregulate the temperature and the magnification, thereby producing a moreuniform membrane. In the biaxial stretching in different directions, amethod of performing the stretching in the MD direction and in the TDdirection, for example, is used widely as a method of obtaining amembrane with little anisotropy. In so-called “wet-process stretching”in which a solvent is used as a plasticizer used for separatorproduction, a simultaneous biaxial stretching process has been usedespecially when polyethylene is used as the resin. Such a simultaneousbiaxial stretching process can provide uniform physical properties ofthe film such as the membrane thickness, the air resistance and thestrength and has been suitable for production of a thin membrane of 10μm or less. However, in the simultaneous biaxial stretching process,since the stretching with a solvent, in other words, “wet-processstretching” is performed in one stage (once), the stretching temperaturehas been regulated only for a small region. In other words, in theconventional wet-method simultaneous biaxial stretching process, a longheating oven was required to prevent the uneven temperatures in theheating oven and to expand the temperature regulation possible rangeduring the stretching, which increased the initial introduction cost.Thus, the expansion of the stretching temperature was difficult,Therefore, in the conventional wet-method simultaneous biaxialstretching process, the membrane production was limited in a relativelynarrow range of temperatures in the heating oven.

Furthermore, wet-process stretching in multi stages, in other words, amethod of performing separately the stretching in one axial direction oftwo axes (two directions) of the MD direction and the TD direction andthe stretching in the other axial direction was also considered.However, such a consideration was mainly about the technique whichfocused on the MD stretching conditions, and few technologicaldevelopments have been made which focus on the influence of eachtemperature in the separate stretching processes in the two axialdirections. As described in detail with respect to the stretchingmagnification, the stretching magnification in each of the MD directionand the TD direction is 12 times or less, and at such a stretchingmagnification, the orientation is mainly in the stretching direction atthe completion stage of the first axial stretching. Therefore, in thismethod, it is considered that the uniformity as a membrane can reach alevel usable as a separator by adjusting the total area stretchingmagnification to 25 times or more in the second axial or subsequentstretching. As the total area stretching magnification becomes higher,the pin puncture strength and the tensile strength are more improved,but the shutdown temperature which affects the safety of the batterytends to rise. The shutdown temperature herein refers to the temperatureat which the pores of the separator are blocked to block the permeationof lithium ions and thus, the battery function is stopped. It isnecessary to increase the strength of the separator compared toconventional ones for a suitable battery production in producing a thinmembrane of 15 μm or less, particularly of 12 μm or less. One method ofincreasing the strength is to increase the total area stretchingmagnification. However, when the shutdown temperature increases andespecially when the total area stretching magnification exceeds 45times, it is more difficult to keep the balance between mechanicalproperties and the safety of the separator. As one method of decreasingthe shutdown temperature, the technique of using a resin having alow-melting point has been developed, but the balance among the airresistance, the strength and the heat shrinkage rate were notsufficient.

For a separator of 15 μm or less, particularly of 12 μm or less having agood balance among the air resistance, the strength and the heatshrinkage rate, to improve the safety as well, the stretchingtemperature conditions were considered for when the stretching isperformed separately and in order in one axial direction of two axes(two directions) of the MD direction and the TD direction (the firstaxial stretching) and then in the other axial direction (the secondaxial stretching). Generally, the shutdown temperature is affectedgreatly by the resin mobility and the pore distribution. Mainly, themeltdown temperature is affected greatly by the melting point and theviscosity of the resin, and when the resin composition is fixed underarbitrary conditions, the meltdown temperature stays constant even ifother production conditions change. The increase in the shutdowntemperature results in a smaller temperature difference between themeltdown temperature and the shutdown temperature. Even if the shutdownfunction shuts off the ion conduction in the case of an abnormal heatgeneration in the battery, when the temperature continues to rise due tothe heat accumulation in the battery, the ion conduction is resumed bythe meltdown of the separator. Thus, the risk of an ignition or the likeincreases. Therefore, it is necessary to lower the shutdown temperatureas much as possible, or widen the difference between the shutdowntemperature and the meltdown temperature.

It is important to lower the shutdown temperature for these reasons. Thereason why the shutdown temperature rises with the increase of the totalarea stretching magnification is assumed that the increase in the totalstretching magnification makes the mobility of the resin constitutingthe separator more restrictive. Accordingly, in the stretching processin which the first axial stretching and the second axial stretching aresequentially carried out (alternatively, the second axial stretching isfurther followed by another stretching at least once in an axialdirection other than the first and second axes), an attempt was made toimprove the mobility of the resin by increasing the stretchingtemperature of the second axial or subsequent stretching with respect tothe that of the first axial stretching; however, contrary to theexpectation, no improvement in the shutdown temperature could beobtained.

Therefore, we studied the influence of the stretching temperature, andfound the following.

That is, we discovered that the shutdown temperature could be decreasedwhen the relationship between the first axial stretching temperature(T1) and the maximal stretching temperature (T2) after the second axialstretching is maintained as in the Equation (2-1). By maintaining therelationship of the Equation (2-1), the shutdown temperature can beadjusted to 140° C. or less, or the temperature difference between theshutdown temperature (SDT) and the meltdown temperature (MDT) can beadjusted to 10° C. or more. Alternatively, when the relationship betweenthe first axial stretching temperature (T1) and the maximal stretchingtemperature (T2) after the second axial stretching is maintained as inEquation (2-1), the temperature difference between the shutdowntemperature and the meltdown temperature can be adjusted to 10° C. ormore while the shutdown temperature is kept under 140° C. or less.β≤T1−T2  (2-1)In Equation (2-1), we discovered that when β is preferably 0° C., morepreferably 2° C., further preferably 5° C., and still further preferably7° C., at least one of the shutdown temperature of 140° C. or less andthe temperature difference of 10° C. or more between the meltdowntemperature and the shutdown temperature could be achieved.

The reason why the shutdown temperature is improved under theseconditions is not clear but can be assumed as follows. As describedabove, it is known that the shutdown temperature is influenced by themobility of the resin and the pore size distribution and, therefore, itcan be understood that the addition of a resin with a low melting pointalso increases the mobility of the resin and thus decreases the shutdowntemperature. The reason for the improvement of the shutdown temperatureby lowering the temperature after the second axial stretching in themultistage stretching process is assumed that, when the second axial orsubsequent stretching is performed while the skeleton of the structureformed by the first axial stretching is maintained, a micro structurestarting from the structure formed by the first axial stretching isformed, thereby lowering the shutdown temperature. We also assume that,when the second axial stretching is performed at a higher temperaturethan in the first axial stretching, the pore-formation and the formationof a new structure by the stretching progress simultaneously, resultingin a broader structure distribution and thus the formation of anon-uniform micropore structure.

At this time, by selecting the stretching conditions that satisfyEquation (2-1), not only the shutdown temperature and the meltdowntemperature can be maintained as described above, but also a finestructure is formed. As a result, a separator which has a smallvariation in permeability after the compression and which is excellentin strength and heat shrinkage rate can be produced. To prevent thedecrease in the air resistance after a treatment at a high temperature(80° C.), β is preferably 0° C., more preferably 2° C., furtherpreferably 5° C., and still further preferably 7° C. In such acondition, the increase in the air resistance after the compression canbe prevented.

The deformation rate in the TD stretching (TDO) can be determined from amembrane production rate and the clip position (site where the film isheld) in the width direction (TD direction). By controlling the railposition in the heating oven in the TD direction, it is possible tocontrol the width-widening rate in the TD direction, that is, thedeformation rate. The deformation rate is desirably controlled at aconstant rate in a region of preferably 80% or more, more preferably 85%or more, further preferably 90% or more of the entire stretching stagesin the TD stretching. The desired deformation rate of the TD stretchingis preferably 200%/second or less, more preferably 150%/second or less,and further preferably 130%/second or less. By setting the deformationrate of the TD stretching to 200%/second, it is possible to suppress theresidual stress in the separator, and achieve a stable production with alow possibility of discontinuation of the production due to a membranerupture or the like. The desired deformation rate of the TD stretchingis preferably 10%/seconds or more, more preferably 15%/seconds or more,and further preferably 45%/seconds or more. By setting the deformationrate of the TD stretching to 10%/second, the investment for theequipment can be suppressed, and the production of economically usefulseparators is possible. The deviation in the deformation rate of the TDstretching (maximal deformation rate−minimal deformation rate) ispreferably 70%/second or less, more preferably 50%/second or less,further preferably 20%/second or less, and the most preferably 5%/secondor less. By controlling the deviation of the deformation rate of the TDOstretching below a certain value, the interaction of ultra highmolecular weight components uniformly develops, and a microporousmembrane which does not greatly deteriorate the air resistance whensubjected to high temperature compression can be obtained.

Due to stretching as described above, cleavage occurs in the higherorder structure formed in the gel-like sheet, resulting in a refinedcrystal phase and the formation of numerous fibrils. Fibrils form athree-dimensionally connected network structure. In addition to theimprovement of the mechanical strength by stretching, the degree ofinteraction between fibrils can be controlled by regulating at least oneof the stretching magnification and the stretching temperature.Therefore, even if a pressure is applied at a temperature of 100° C. orless, a structure whose performance hardly changes is obtained. Further,by performing the above stretching, since the shape is retained even ata high temperature, the insulation property can be easily maintained.Thus, the resulting microporous membrane is suitable for a separator ofa battery, for example.

Washing and Drying

The stretched sheet thus obtained is subjected to a conventionaltechnique, for example, a method described in WO2008-016174 to wash andremove the diluent, followed by drying, and thus a dried microporousplastic film is obtained. In obtaining a microporous plastic film, thestretched sheet may be reheated in the dry-process stretching step(second stretching step) after the washing step and re-stretched. There-stretching step may be either by a roller type or a tenter type. Itis also possible to adjust physical properties and remove residualstrain by performing a heat treatment in the same step. By using thetemperature condition according to Equation (3) as the dry-processstretching temperature D (T), the solid heat shrinkage rate is furtherimproved. Specifically, the use of the condition of Equation (3) candecrease the heat shrinkage rate at 105° C., and a desired heatshrinkage rate is preferably 8% or less, more preferably 6% or less, andfurther preferably 5% or less in both the MD direction and the TDdirection.SDT−D(T)≤γ  (3)

“SDT” refers to “the shutdown temperature” and the measurement methodthereof is explained later. When γ is preferably 12° C., more preferably10° C., and further preferably 8° C., the solid heat shrinkage rate canbe suppressed.

That is, various experiments are carried out in advance and data areobtained to figure out what kind of correlation exists between theshutdown temperature and the dry-process stretching temperature, andbased on the acquired data, the dry-process stretching temperature isroughly set so that the shutdown temperature is, for example, around140° C. While the membrane formation is continued, the shutdowntemperature of the microporous membrane which has been already wound ismeasured, and the dry-process stretching temperature is adjusted tosatisfy Equation (3) as described above. A microporous membrane with anexcellent heat shrinkage rate can be thus obtained.

To produce a microporous membrane which shows a small deterioration ofthe air resistance after the compression, among Equations (1-1), (2-1)and (3) explained above, Equation (1-1) is preferably satisfied becausethe resulting microporous membrane has a small air resistance variationafter a pressurization at 80° C. and under 4 MPa for 10 minutes, after apressurization at 60° C. and under 4 MPa for 10 minutes, and after theincrease of the heating temperature from 60° C. to 80° C. When Equation(2-1) is satisfied in addition to Equation (1-1), the deterioration ofthe air resistance after the compression is suppressed more than whenonly Equation (1-1) is satisfied, and it is also possible to decreasethe shutdown temperature (or increase the temperature difference betweenthe shutdown temperature and the meltdown temperature).

Moreover, Equation (2) is preferably satisfied simultaneously with theachievement of 2% or more of an ultra high molecular weight polyethyleneor 5% or more of a polyethylene component having a molecular weight of1,000,000 or more, because the microporous membrane thus obtained has atleast one of the shutdown temperature of 140° C. or less or thedifference of 10° C. or less between the shutdown temperature and themeltdown temperature.

To produce a microporous membrane with a high strength, the condition ofcontaining 2% or more of the ultra high molecular weight polyethylene or5% or more of a polyethylene component having a molecular weight of1,000,000 or more and the condition of the total surface stretchingmagnification of 25 times or more before the extraction of the solventare preferably satisfied at the same time because the resultingmicroporous membrane shows improved pin puncture strength and tensilestrength, such as the pin puncture strength of 4000 mN (12 μmconversion). To control the tensile rupture elongation and improve theprocessability for when a battery is assembled using the microporousmembrane or the surface of the microporous membrane is coated, the totalsurface stretching magnification before the solvent extraction of 40times or more is preferred because it is possible to obtain amicroporous membrane having an average tensile rupture elongation (thesquare root of the product of the tensile rupture elongation in the MDdirection and the tensile rupture elongation in the TD directions) of130% or less.

Equation (3) is preferably satisfied because it is possible to obtain amicroporous membrane having a solid heat shrinkage rate (for example,heat shrinkage rate after a heat treatment at 105° C. for 8 hours) of 5%or less for each of MD and TD.

As a method of controlling the average pore size of the microporousmembrane, when an ultra high molecular weight polyethylene is containedin an amount of 2% or more or a polyethylene component having amolecular weight of 1,000,000 or more is contained in an amount of 5% ormore, a microporous membrane in which the maximal pore size ispreferably 0.15 μm or less and the average pore size is 0.1 μm or less,and more preferably, the maximal pore size is 60 nm or less and theaverage pore size is 50 nm or less, can be obtained.

Preferably, the microporous membrane obtained herein has acharacteristic in which a ladder-like structure is dominant in thesubmicron region and a hybrid structure having a three-dimensionalnetwork structure is dominant in the micron region. The “submicronregion” refers to a structure size that can be confirmed under theobservation with a 4 μm square field of view (4 μm×4 μm) by AFM (atomicforce microscope) or the like. On the other hand, the “micron region”refers to a structure size that can be confirmed under the observationwith a field of view of 12 μm square or more (12 μm×12 μm) by AFM or thelike. The “ladder-like structure” defines a case where structuresbetween adjacent fibrils (arrangement structure between adjacentfibrils) are arranged in a positional relationship which is close toorthogonal to each other. The “three-dimensional network structure”defines when fibrils take a three-dimensional network-like structure.“Dominant” indicates when a corresponding structure is observed in 50%or more of the area in the observation field. Therefore, under theobservation of a fibril at an arbitrary position, when fibrilsorthogonal to (intersecting) this fibril can be dominantly confirmed,they are referred to as the aforementioned “ladder-like structure”. Whenfibrils branching from the fibril at the arbitrary position aredominantly confirmed, they are called “three-dimensional networkstructure”. The “micron region” is formed by “submicron regions”, butwhen the “micron region” is observed, we believe that the features of alarger and thicker structure become dominant and are observed as thestructure difference as described above.

FIGS. 1 and 2 show AFM photos obtained from the microporous membrane ofthe present invention showing such a structure. FIG. 1 shows a phototaken in the micron region as described above, and FIG. 2 shows a phototaken in the submicron region. Hereinafter, the longitudinal directionin the Figures is the MD direction and the transverse direction is theTD direction. As described in detail above, fine fibrils branch out froma middle portion of a specific fibril and extend to form athree-dimensional structure in the micron region while, in the submicronregion, each fibril is arranged in a way that other fibrils intersectwith the specific fibril to form a ladder-like structure. As a preferredmicroporous membrane, any one of the surfaces has the hybrid structuredescribed above, and more preferably, the both sides have the abovehybrid structure. The ratio of regions having characteristic structuresmay be different between both the surface layers.

The structure of the microporous membrane produced by a “simultaneousstretching” method in which an unstretched sheet is stretched in the MDdirection and in the TD direction simultaneously is different from themicroporous membrane because the “three-dimensional network structure”is formed in both of the submicron and micron regions. Specifically, theAFM photos of a microporous membrane produced by such a simultaneousstretching method are shown in FIG. 3 (the micron region) and FIG. 4(the submicron region), and certain fibrils branch off in a net-likeshape to form a three-dimensional structure in both Figures.

In a conventional stepwise biaxial stretching method in which anunstretched sheet is stretched gradually in the MD and TD directions,although the submicron region shows a ladder-like structure, the micronregion shows a structure in which the fibrils are arranged selectivelyin the MD direction or in the TD direction (ladder-like structure). TheAFM photos of a microporous membrane produced by a conventional stepwisebiaxial stretching method are shown in FIG. 5 (micron region) and FIG. 6(submicron region). The fibrils are arranged in a ladder form in bothpictures (the fibrils are not arranged randomly, but oriented in onedirection). Therefore, the microporous membrane has a structuredifferent not only from that of microporous membranes produced by thesimultaneous biaxial stretching method but also from that of microporousmembranes obtained by the stepwise biaxial stretching process which hasbeen known so far.

In summary, the microporous membrane forms a three-dimensional networkstructure formed by a large number of ladder-like structures. That is,the microporous membrane shows a ladder-like structure from amicroscopic view, but as the ladder-like structures accumulate, eachladder-like structure is arranged so that a three-dimensional networkstructure is formed. On the other hand, the microporous membranesproduced by the conventional stepwise biaxial stretching method has aladder-like structure as a result of the accumulation of ladder-likestructures while the microporous membranes produced by the conventionalsimultaneous biaxial stretching method has a three-dimensional networkstructure as a result of the accumulation of three-dimensional networkstructures. Therefore, an microporous membrane can possess a highlyuniform and fine structure inherent in the microporous membranesproduced by the simultaneous stretching method and a structure capableof achieving high permeability and strength, which is inherent in themicroporous membranes produced by the stepwise biaxial stretchingmethod. With such a structure, while high permeability, high strengthand low heat shrinkage are exhibited, the safety function(low-temperature SDT and high-temperature MDT) can be achieved withoutdeteriorating the ion conductivity even under high temperature andcompression conditions in the battery. Therefore, as can be seen fromExamples, compared with the comparative examples, remarkablecharacteristics in terms of physical properties can be obtained.

Further, depending on the application of the microporous membrane thusobtained, a surface treatment such as corona discharge or a functionalcoating such as heat resistant particles may be applied to the surfaceof the microporous plastic film.

The polyolefin microporous membrane according to a preferred example hasthe following physical properties.

(1) Membrane Thickness (μm)

The membrane thickness of the polyolefin microporous membrane ispreferably 3 to 15 μm, more preferably 3 to 12 μm, and furtherpreferably 5 to 12 μm as the density and capacity of batteries have beenincreasing in recent years. The membrane thickness of 3 μm or more canresult in a separator which assures the insulation property.

(2) Bubble Point (BP), Pore Size and Average Pore Size (average FlowHole Pore Size) (nm)

The polyolefin microporous membrane has a maximal pore size ofpreferably 0.15 μm or less, more preferably of 0.12 μm or less, furtherpreferably of 0.1 μm or less, and still further preferably of 0.06 μm orless. The maximal pore size is determined from bubble points (BP)obtained by a palm porometer. The average pore size determined with apalm porometer is preferably 0.1 μm or less, more preferably of 0.08 μmor less, further preferably of 0.06 μm or less, still further preferablyof 0.05 μm or less, and still further preferably of 0.039 μm or less. Asmaller pore size of the whole membrane makes the pores more resistantto crashes and reduces the variation in the membrane thickness and theair resistance.

(3) Air Resistance (sec/100 cm³)

The air resistance (Gurley value) is preferably 400 sec/100 cm³ or less.With the air resistance of 400 sec/100 cm³ or less, when the microporousmembrane is used in a battery, good ion conductivity is obtained. Theair resistance can be adjusted by the stretching temperature andmagnification before the extraction of the solvent, the dry-processstretching temperature and magnification after washing, and the resincomposition.

(4) Porosity (%)

The porosity is preferably 25 to 80%. When the porosity is 25% or more,good air resistance can be obtained. When the porosity is 80% or less,the strength in the battery in which the microporous membrane is used asa separator is sufficient, and the short circuit can be prevented. Theporosity is more preferably 25 to 60%, and further preferably 25 to 50%.The porosity in such a range is preferred because the pores of theseparator are unlikely to be crashed during the compression.

(5) The 12 μm-Corresponding Pin Puncture Strength (mN)

The pin puncture strength corresponding to 12 μm is 4000 mN (408 gf) ormore, preferably 4500 mN, and more preferably 4900 mN or more. With thepin puncture strength corresponding to 12 μm is 4000 mN or more, whenthe microporous membrane is incorporated in a battery as a separator forbatteries, especially in the case of a thin membrane of 15 μm or less,the short circuit between electrodes can be prevented.

(6) Tensile Rupture Strength (MPa)

The tensile rupture strength is preferably 80 MPa or more either in theMD direction and in the TD direction. The tensile rupture strength inthis range can reduce the risk of the rupture of the membrane. Thetensile rupture strength in the MD direction is preferably 110 MPa ormore, more preferably 140 MPa or more, and further preferably 210 MPa ormore. The tensile rupture strength in the TD direction is preferably 120MPa or more, more preferably 170 MPa or more, and further preferably 180MPa or more. When the tensile rupture strength is in the above preferredrange, the membrane is more resistant to rupture even when pressedthermally under a high pressure in the production step of the battery,and thus the pores are unlikely to be crashed.

(7) Tensile Rupture Elongation (%), Average Tensile Rupture Elongation(%)

The tensile rupture elongation is 40% or more either in the MD directionand in the TD direction. This reduces the possibility of the rupture ofthe membrane of the separator during the battery production and whenexternal force acts on the battery. The average tensile ruptureelongation determined by the equation explained later is preferably 130%or less, more preferably 120% or less, further preferably 110% or less,and in this range, the residual strain after the winding can be lowered,and excellent processability is obtained.

(8) Heat Shrinkage Rate After the Exposure to a Temperature of 105° C.for 8 Hours (Solid Heat Shrinkage Rate) (%)

The heat shrinkage rate after the exposure to a temperature of 105° C.for 8 hours is 5% or less in both of the MD direction and TD direction.When the heat shrinkage rate is 5% or less, even in the use of themicroporous membrane as a separator for a large lithium battery, a shortcircuit between the electrodes can be prevented since the end portionsof the separator retracts toward the center at the time of heatgeneration. Therefore, to prevent the short circuit between theelectrodes even when the battery generates heat, the heat shrinkage rateis preferably 8% or less, more preferably 6% or less, and furtherpreferably 5% or less in both of the MD direction and TD direction. Theheat shrinkage rate is still further preferably less than 5%. The heatshrinkage rate is preferably 4% or less in both of the MD direction andTD direction, and especially in the MD direction.

(9) Shutdown Temperature and Meltdown Temperature (° C.)

The shutdown temperature is preferably 145° C. or less, more preferably143° C. or less, and further preferably 140° C. or less. With theshutdown temperature in this range, pores close at a lower temperature,thereby blocking the mobility of lithium ions, and thus the batteryfunction can be safely stopped.

The meltdown temperature is preferably 145° C. or more, more preferably147° C. or more, and further preferably 149° C. or more. A largerdifference between the meltdown temperature and the shutdown temperatureis preferred. With a larger difference between the meltdown temperatureand the shutdown temperature, when the flow of lithium ions stops duringthe abnormal heat generation in the battery, the battery function canstop without resuming the lithium ion conduction even in the case ofoverheating (overshoot) of the temperature in the battery. Thedifference between the meltdown temperature and the shutdown temperatureis preferably 8° C. or more, more preferably 10° C. or more, and furtherpreferably 11° C. or more.

Physical properties associated with the heat compression test aredescribed below. The upper limit of the internal pressure of the batterythat can occur when high capacity electrodes are applied was set at 4.0MPa. The heating temperatures were 60° C. and 80° C. Between these, 60°C. is assumed to be the temperature range inside the battery when thebattery is used at a low rate (the use at less than 1 C, wherein 1 C isthe discharge rate for the exhaustion within 1 hour), and 80° C. isassumed to be the maximal temperature that can occur inside the batterywhen a rapid charge and discharge state of 1 C or more is performed.

(10) Thickness Variation After the Heat Compression (%)

As an example, a pressurization test was performed assuming theconditions of 80° C. and 4.0 MPa as the upper limit of temperature andpressure that can occur (reach) at a high output in a large battery of 5Ah or more. The membrane thickness variation after the heat compressionunder a pressure of 4.0 MPa at 80° C. for 10 minutes (see the equationbelow) is, when the membrane thickness before the compression isconsidered as 100%, preferably 85% or more, more preferably 87% or more,and further preferably 89% or more. With the membrane thicknessvariation of 85% or more, when the microporous membrane is used as abattery separator, the volume of the separator after the charge anddischarge shows a small variation. Therefore, the number of electrodesto be used can be increased at during the battery assembly, leading tothe maximization of the battery capacity.

The membrane thickness variation after the heat compression under apressure of 4.0 MPa at 60° C. for 10 minutes (the same equation asabove) is, when the membrane thickness before the compression isconsidered as 100%, preferably 90% or more, more preferably 91% or more,and further preferably 96% or more.

(11) Air Resistance Variation After the Heat Compression (%)

The air resistance variation (60° C.) after the heat compression under apressure of 4.0 MPa at 60° C. for 10 minutes (the variation of Gurleyvalues (sec/100 cm³) before and after the heat compression, see theequation below) is preferably 148% or less, more preferably 145% orless, and further preferably 140% or less. When the air resistancevariation after the heat compression under a pressure of 4.0 MPa at 60°C. for 10 minutes is in this range, a battery which suppresses thedeterioration of the ion permeability in the temperature range insidethe battery (about 60° C.) which is assumed when the battery is used ata low rate (the use at less than 1 C, wherein 1 C is the discharge ratefor the exhaustion within 1 hour) can be formed. Depending on thebattery system to be used, when the air resistance variation after theheat compression at 60° C. for 10 minutes is 148% or less (600 sec/100cm³ or less in the air resistance), the deterioration of the cycleproperty, which is the battery life, is suppressed.

On the other hand, the air resistance variation (80° C.) after the heatcompression under a pressure of 4.0 MPa at 80° C. for 10 minutes (thevariation of Gurley values (sec/100 cm³) before and after the heatcompression) is preferably 200% or less, more preferably 190% or less,and further preferably 180% or less. When the air resistance variationafter the heat compression at 80° C. for 10 minutes is 200% or less,especially in use as a separator for a large battery, the deteriorationof the battery performance can be suppressed in the temperature rangewhich can occur partially for a short time inside the battery during therapid charge and discharge.

The variation ratio of the air resistance variation (80° C.) to the airresistance variation (60° C.) (80° C./60° C.), which is defined by thefollowing equation, is preferably 140% or less, more preferably 135% orless, and further preferably 130% or less. This ratio is furtherpreferably. 122% or less.

The variation ratio (80° C./60° C.)=(the air resistance variation (80°C.)/the air resistance variation (60° C.))×100

The smaller this ratio is, the less deterioration of the separatorperformance occurs when the rapid charge and discharge is sporadicallyperformed during normal use. That is, if the ratio is 140% or less, thedeterioration of the cycle property can be suppressed even after therapid charge and discharge and, thus, the irreversible deterioration ofthe impedance of the separator, which is a measure of flowability oflithium ions, can be prevented.

The membrane thickness variation (%) after the heat compression and theair resistance variation (%) after the heat compression are easilyaffected by the crystal orientation, the structure of pores of themembrane, the heat shrinkage rate and the like. In particular, amicroporous membrane with a small variation in the air resistance can beobtained when the interaction formed by the polyethylene molecularweight component having a molecular weight of 1,000,000 or more occursuniformly in the microporous membrane. As a means of achieving this, inaddition to the control of the stretching magnification and thestretching temperature at the stretching stage under certain conditions,the deformation rate during stretching can be maintained within aconstant rate fluctuation range to control uniformly the interactionstructure formed by the ultra high molecular weight polyethylenecomponent.

EXAMPLES

Test Methods

Our membranes, batteries and methods will be explained in more detail byway of the following Examples, but this disclosure is not limited tothese Examples. The physical properties of the polyolefin microporousmembrane were measured by the following methods.

(1) Membrane Thickness (μm)

The membrane thickness of 5 points within the range of 95 mm×95 mm ofthe microporous membrane was measured with a contact thickness meter(Litematic manufactured by Mitutoyo Corporation), and the average valueof the membrane thickness was obtained.

(2) Average Pore Size (Average Flow Rate Pore Size) and Maximal (bubblePoint (BP)) Pore Size (nm)

The average pore size (average flow pore size) and maximal (bubble point(BP)) pore size (nm) of the polyolefin microporous membrane weremeasured as follows.

A palm porometer manufactured by Porous Materials Inc. (trade name,model: CFP-1500A) was used to carry out the measurement in the order ofDry-up and Wet-up. For Wet-up, a pressure was applied to the polyolefinmicroporous membrane sufficiently soaked with Galwick (trade name) witha known surface tension, and the pore size converted from the pressureat which air started penetrating was used as the maximal pore size. Forthe average flow rate size, the pore size was converted from thepressure at the point where the pressure and flow rate curve showing theslope of ½ from the Dry-up measurement and the curve from the Wet-upmeasurement intersect. The following equation was used to convert thepressure to the pore size.d=C·γ/Pwherein d (μm) is the pore size of the microporous membrane, γ(dynes/cm) is a surface tension of the liquid, P (Pa) is the pressure,and C is the pressure constant (2860).(3) Air Resistance (Sec/100 cm³)

The air resistance (Gurley value) was measured in accordance with JISP8117.

(4) Porosity (%)

The porosity was calculated from the following equation, using the massw1 of the microporous membrane and the mass w2 of a pore-free membraneof the same size made of the same polyethylene composition as themicroporous membrane.Porosity (%)=(w2−w1)/w2×100.(5) Pin puncture strength (mN) and 12 μm-corresponding pin puncturestrength (mN/12 μm)

The maximal load value (P1) when a polyolefin microporous membrane waspunctured with a needle having a diameter of 1 mm (tip: 0.5 mmR) at aspeed of 2 mm/sec was measured as the pin puncture strength. The 12μm-corresponding pin puncture strength (P2) was converted to thecorresponding value in the membrane thickness T1 (μm), using thefollowing equation.P2=(P1×12)/T1.(6) Tensile Rupture Strength (MPa)

The tensile rupture strength was measured according to ASTM D882 using astrip test piece having a width of 10 mm.

(7) Tensile Rupture Elongation (%) and Average Tensile RuptureElongation (%)

The tensile rupture elongation was determined by taking 3 strips with awidth of 10 mm from the central portion of the polyolefin microporousmembrane in the width direction and calculating the average value of themeasurement results by ASTM D882 for each. The average tensile ruptureelongation (L_(A); %) obtained from the tensile rupture elongation inthe MD (L_(M)) and TD (L_(T)) directions was determined by the followingequation.L _(A)=(L _(M) ×L _(T))^(0.5).(8) Heat Shrinkage Rate After the Exposure to a Temperature of 105° C.for 8 Hours (%)

For the heat shrinkage rate, the shrinkage rate in the MD direction andthe TD direction after the exposure of the microporous membrane at 105°C. for 8 hours was individually measured three times, and the averagevalue was calculated.

(9) Shutdown Temperature (SDT) and Meltdown Temperature (MDT) (° C.)

The shutdown temperature and the meltdown temperature were measured bythe method disclosed in WO2007/052663. According to this method, themicroporous membrane is exposed to an atmosphere at 30° C., and thetemperature is raised at 5° C./min, during which the air resistance ofthe membrane is measured. The temperature at which the air resistance(Gurley value) of the microporous membrane first exceeds 100,000seconds/100 cm³ was defined as the shutdown temperature of themicroporous membrane. The meltdown temperature was defined as thetemperature at which the air resistance which was measured while thetemperature continued to rise after reaching the shutdown temperaturereached again 100,000 seconds/100 cm³. The air resistance of themicroporous membrane was measured in accordance with JIS P8117 using anair resistance meter (EGO-1T, manufactured by Asahi Seiko Co., Ltd.).

(10) Membrane Thickness Variation After the Heat Compression (%)

The membrane thickness was measured with a contact thickness meter(manufactured by Mitutoyo Corporation). A polyolefin microporousmembrane is sandwiched between a pair of press plates having a highlysmooth surface and subjected to heat compression by a press machineunder a pressure of 4.0 MPa at 80° C. (or 60° C.) for 10 minutes. Avalue obtained by dividing the membrane thickness after the compressionby the membrane thickness before the compression is expressed inpercentage as a membrane thickness variation (%) (see the equationbelow). Three points from the central portion of the polyolefinmicroporous membrane in the width direction were measured for themembrane thickness, and the average value of the measurement resultswere calculated.Membrane thickness variation (%)=(membrane thickness aftertreatment)/(membrane thickness before treatment)×100The “treatment” in the above equation means a treatment ofpressurization and compression under the above-described conditions.(11) Air Resistance Variation by the Heat Compression Treatment (%)

A polyolefin microporous membrane was subjected to heat compressionunder the same conditions as in the above (10), and the value obtainedby dividing the air resistance after the heat compression by the airresistance before the heat compression was expressed in percentage asthe air resistance variation (%) (see the equation below). Three pointsfrom the central portion of the polyolefin microporous membrane in thewidth direction were measured for the air resistance, and the averagevalue was calculated.Air resistance variation(80° C.)(%)=the air resistance after thepressurization treatment at 80° C./the air resistance prior to thepressurization treatment×100.Air resistance variation(60° C.)=the air resistance after thepressurization treatment at 60° C./the air resistance prior to thepressurization treatment×100.The variation ratio of the air resistance variation (80° C.) to the airresistance variation (60° C.) (80° C./60° C.) was determined from thefollowing equation.The variation ratio(80° C./60° C.)=(the air resistance variation(80°C.)/the air resistance variation (60° C.))×100(12) Impedance (Ω·cm²)

An impedance measuring device (SI 1250, SI 1287, manufactured bySolartron Metrology Ltd) was used to measure the impedance. Betweenelectrodes in which a Ni foil (30 mm×20 mm) was provided on a glassplate (50 mm (W)×80 mm (L)×3 mm (T)), a microporous membrane (30 mm(W)×20 mm (L)) and about 0.02 ml of a typical electrolyte consisting of,using LiPF₆ of 1 mol/L, a lithium salt, ethylene carbonate (EC) andethylmethyl carbonate (EMC) (EC:EMC=40:60 VOL %) were placed, and 1.0 kVwas applied (constant voltage), and the value after 10 seconds wasdefined as electric resistance (Ω·cm²). The measured value at roomtemperature was considered as 100%, and the impedance value after theheat compression treatment was obtained as a relative value.

(13) Shutdown Behavior and Meltdown Behavior Under Rapid Heating

Using the device described in (12) above, the above cell is placed in anoven. The temperature inside the oven was increased from a roomtemperature (25° C.) to 200° C. in 30 minutes, and the impedance wascontinuously measured. As the cell temperature was measured, thetemperature at which the impedance first reached 1.0×10⁴ (Ω·cm²) wasconsidered as the shutdown temperature, and while the temperatureincrease was continued, the duration during which 1.0×10⁴(Ω·cm²) wasmaintained was measured. The evaluation was as follows: 1) remarkablyexcellent for 60 seconds or more ({circle around (∘)}), 2) excellent for45 seconds or more (∘), 3) standard for 30 seconds or more (Δ). On theother hand, it was evaluated as 4) inappropriate (×) for less than 30seconds, and as 5) not able to contribute to the battery safety when theshutdown state was not reached. Such a test was carried out twice foreach Example and each Comparative Example (limited to the samples judgedto be testable) to obtain an average. In Tables 3 and 4 described later,these impedance evaluation results were expressed as “impedanceretention test result”.

(14) Polyethylene Molecular Weight and Molecular Weight DistributionMeasurement

The Mw and Mw/Mn of UHMWPE and HDPE each were measured by gel permeationchromatography (GPC) under the following conditions.

-   Measurement apparatus: GPC-150C available from Waters Corporation,-   Column: Shodex UT 806 M available from Showa Denko K.K.,-   Column temperature: 135° C.,-   Solvent (mobile phase): o-dichlorobenzene,-   Solvent flow rate: 1.0 ml/min,-   Sample concentration: 0.1% by weight (dissolved at 135° C. for 1    hour),-   Injection volume: 500 μl,-   Detector: differential refractometer available from Waters    Corporation, and-   Calibration curve: it was prepared from a calibration curve of a    standard polystyrene monodisperse sample using a predetermined    conversion constant.    (15) Observation of the Surface of the Microporous Membrane

The surface of the microporous membrane in the submicron region or themicron region can be observed with a commercially available scanningprobe microscope. As an example of such a scanning probe microscope, ameasurement example using SPA-500 manufactured by HitachiHigh-Technologies will be described below. Specifically, the surface ofthe microporous membrane can be observed in the DFM mode of such adevice. A sample (microporous membrane) is fixed to the sample stagewith a carbon tape, and the SI-DF 40 for DMF can be used as thecantilever. Then, the amplitude attenuation rate is set to −0.25 to−0.3, the scanning frequency is set to 0.5 to 1.0 Hz, and the I gain andthe P gain are individually adjusted to observe the surface. A typicalfield size to be used can be 4 μm square and 12 μm square. The“ladder-like structure” refers to a structure where structures formedbetween adjacent fibrils (arrangement structure between adjacentfibrils) are arranged in a positional relationship which is close toorthogonal to each other. The “three-dimensional network structure”defines the case where the fibrils take a three-dimensionallynetwork-like structure. “Dominant” indicates when a correspondingstructure is observed in 50% or more of the area in the observationfield. Therefore, under the observation of a fibril at an arbitraryposition, when fibrils orthogonal to (intersecting) this fibril could bedominantly confirmed, they were referred to as the aforementioned“ladder-like structure”. When fibrils branching from the fibril at thearbitrary position were dominantly confirmed, they were called“three-dimensional network structure”.

Example 1

Production of the Polyolefin Microporous Membrane

To 100 parts by mass of a composition consisting of 30% by mass of ultrahigh molecular weight polyethylene having a mass average molecularweight of 2.7×10⁶ and 70% by mass of high density polyethylene having amass average molecular weight of 2.6×10⁵, 0.375 parts by mass oftetrakis[methylene-3-(3,5-ditertiarybutyl-4-hydroxyphenyl)-propionate]methanewas dry-blended to prepare a polyethylene composition. The obtainedpolyethylene composition was charged in an amount of 30 parts by weightinto a twin-screw extruder. Further, 70 parts by weight of liquidparaffin was fed from the side feeder of the twin-screw extruder andsubjected to melt blending. Thus, a polyethylene resin solution wasprepared in the extruder. Subsequently, a polyethylene resin solutionwas extruded at 190° C. from a die installed at the tip of the extruder,and taken out by a cooling roll with the internal cooling watertemperature kept at 25° C. to form an unstretched gel-like sheet.

The obtained unstretched gel-like sheet was passed through a group offour preheating rolls to raise the temperature of the sheet surface to115° C. For a longitudinal stretching (MDO) roll, a metal roll with awidth of 1000 mm, a diameter of 300 mm, and a hard chromium plating (thesurface roughness: 0.5 S) was used. The surface temperature of eachlongitudinal stretching roll was 123° C., and the range of temperaturefluctuation was +2° C. or less. By controlling the peripheral speedratio of each roll such that the rotation speed of each stretching rollof the longitudinal stretching apparatus was faster downward, thegel-like sheet was stretched in the machine direction by 1.3/1.8/3.5times in three divided stages with a total magnification of 8.2 times.Then, the sheet was passed through four cooling rolls and the sheettemperature was cooled to 50° C. Thus, a longitudinally stretchedgel-like sheet was formed.

Both ends of the resulting longitudinally stretched gel-like sheet weregripped with clips, and the sheet temperature was raised at a preheatingtemperature of 114° C. in a tenter divided into 20 zones, and then thesheet was stretched in the transverse direction at 113° C. by 8.3 timesand treated at a thermal fixation temperature of 85° C. Thus, abiaxially stretched gel-like sheet was formed. The interval between theclips in the direction of the traveling sheet was set to 5 mm from theentrance of the tenter to the exit. The wind speed fluctuation width ofhot air in the width direction in the tenter was adjusted to be 3 m/secor less. The average deformation rate in the transverse stretching (TDO)region was 52%/second, and the deviation between the maximal and minimaldeformation rate was 2%/second. The obtained biaxially stretchedgel-like sheet was cooled to 30° C., and then liquid paraffin wasremoved in a methylene chloride washing tank controlled at 25° C., anddried in a drying oven adjusted to 60° C.

The obtained dried sheet was heated to 132° C. in a re-stretchingdevice, re-stretched to have a lateral magnification of 1.7 times withrespect to the entrance width of the re-stretching device, and thensubjected to a heat treatment such that the transverse magnificationwould be 1.6 times with respect to the entrance width of there-stretching device. Then, the obtained sheet was subjected to a heattreatment for 20 seconds to obtain a polyolefin microporous membranehaving a thickness of 12 μm.

The production conditions of the microporous membrane in Example 1 areshown in Tables 1 and 2, and the physical properties obtained from thismicroporous membrane are shown in Tables 3 and 4.

Examples 2 to 4

Microporous membranes were obtained in the same way as in Example 1except that the composition of the resins, and the like were changedaccording to the conditions described in Tables 1 and 2.

Comparative Example 1

To 100 parts by mass of a composition consisting of 40% by mass of ultrahigh molecular weight polyethylene having a mass average molecularweight of 2.7×10⁶ and 60% by mass of high density polyethylene having amass average molecular weight of 2.6×10⁵, 0.375 parts by mass oftetrakis[methylene-3-(3,5-ditertiarybutyl-4-hydroxyphenyl)-propionate]methanewas dry-blended to prepare a polyethylene composition. The obtainedpolyethylene composition was charged in an amount of 28 parts by weightinto a twin-screw extruder. Further, 72 parts by weight of liquidparaffin was fed from the side feeder of the twin-screw extruder andsubjected to melt blending. Thus, a polyethylene resin solution wasprepared in the extruder. Subsequently, a polyethylene resin solutionwas extruded at 190° C. from a die installed at the tip of the extruder,and taken out by a cooling roll with the internal cooling watertemperature kept at 25° C. to form an unstretched gel-like sheet.

The obtained unstretched gel-like sheet was introduced into asimultaneous biaxial stretching machine and stretched by 5×5 times inthe sheet transport direction (MD) and in the sheet width direction(TD). The temperature of the preheating/stretching/thermal fixation wasadjusted to 117/117/100° C. The obtained biaxially stretched gel-likesheet was cooled to 30° C., and then liquid paraffin was removed in amethylene chloride washing tank controlled at 25° C., and dried in adrying oven adjusted to 60° C.

The obtained dried sheet was heated to 127° C. in a re-stretchingdevice, re-stretched to have a lateral magnification of 1.4 times withrespect to the entrance width of the re-stretching device, and thensubjected to a heat treatment such that the transverse magnificationwould be 1.2 times with respect to the entrance width of there-stretching device. Then, the obtained film was subjected to a heattreatment for 20 seconds to obtain a polyolefin microporous membranehaving a thickness of 11 μm.

Comparative Example 2

A microporous membrane was obtained in the same way as in ComparativeExample 1 except that the conditions were changed as described in Tables1 and 2.

Comparative Examples 3, 4

Microporous membranes were obtained in the same way as in Example 1except that the composition of the resins, and the like were changedaccording to the conditions described in Tables 1 and 2.

Comparative Example 5

A microporous membrane was obtained from the same polyethylenecomposition as in Example 1 under the conditions described in Tables 1and 2, The heat shrinkage rate at 105° C. was high both in the MDdirection and TD direction. The evaluation was canceled because of theinferior safety.

Comparative Example 6

The membrane production experiment was attempted by changing thelongitudinal stretching temperature of Example 1 to 80/85° C. inpreheating/stretching steps, respectively. In the longitudinalstretching step, a behavior of the sheet shifting in the width directionof the stretching roll over time (generally referred to as “meandering”)was observed, and thus, a stable membrane production could not beperformed.

Comparative Example 7

The membrane production experiment was attempted by changing thelongitudinal stretching temperature of Example 2 to 130/135° C. inpreheating/stretching steps, respectively. A phenomenon in which liquidparaffin which was a solvent flew out from the sheet was observed duringthe preheating stage, and meandering occurred before the stretchingroll. A stable membrane production was difficult.

Comparative Example 8

A membrane was produced under the same conditions as in Example 1 exceptthat TDO preheating/stretching temperatures were changed to 85/90° C.,respectively. Since the stretching temperature was low, the stretchingtension was increased, and a phenomenon that the film was detached fromthe holding clips during the TDO stretching. As a result, a microporousmembrane could not be obtained.

Comparative Example 9

A membrane was produced under the same conditions as in Example 1 exceptthat TDO preheating/stretching temperatures were changed to 125/133° C.,respectively. The TDO stretching temperature was so high that thestretching behavior varied in the TD direction, resulting in an unevenfilm thickness, and a stable membrane production was impossible.

Comparative Example 10

A membrane was produced under the conditions in Tables 1 and 2. Theresulting separator has an SDT or more than 140° C., and the differencefrom the meltdown temperature was less than 10° C. Therefore, the safetywas considered inferior, and the evaluation was canceled.

Comparative Example 11

A membrane was produced under the conditions in Tables 1 and 2. Afterthe longitudinal stretching roll, the sheet thickness varied over timein the MD direction after the MDO stretching. The resulting separatorhad uneven thickness in the MD direction, and wrinkles and sagging wereobserved. The resulting separator was unsuitable for use as a separator.

Comparative Example 12

A membrane was produced under the same conditions as in Example 1 exceptthat the TDO magnification was changed to 4.9 times. Since thedistribution of the thickness in the TD direction was unlikely to beeven, and the air resistance, the strength and the heat shrinkage ratevaried in the TD direction, the evaluation as a separator was difficult.

Comparative Example 13

A membrane was stretched under the same conditions as in Example 1except that the TDO stretching magnification was changed to 11 times.Due to the high stretching magnification, the membrane was rupturedduring the TDO stretching process, and it was impossible to obtainstably a separator.

Comparative Example 14

A membrane was produced under the conditions in Tables 1 and 2. Theseparator obtained by increasing the MDO stretching magnification wasexcellent in the strength and the heat shrinkage rate, but thecompression resistance turned out to be inferior.

Based on Tables 3 and 4 in which the above results are described,Examples 1 to 4 show that the obtained separator showed excellentpermeability and a small membrane thickness variation even after theexposure to a high temperature and a high pressure, and were excellentin strength and heat shrinkage rate required as a separator.Furthermore, by controlling SDT and MDT, which are a measure of safety,a high insulation state after the shutdown (impedance maintained at1×10⁴ (Ω·cm²)) could be achieved over a long period of time.

TABLE 1 Compar- ative Example 1 Example 2 Example 3 Example 4 Example 1Resin Ultra high Content proportion (%) 30 20 20 30 40 compositionpolyethylene (Mw (×10³), f (≥1M)) 2700, 46% 2700, 46% 2700, 46% 2700,46% 2700, 46% High density Content proportion (%) 70 80 80 70 60polyethylene (Mw (×10³), f (≥1M))  260, 5.0%  260, 5.0%  260, 5.0%  260,5.0%  260, 5.0% Solid content 30 30 30 30 28 concentration (%)Stretching Stepwise MDO Preheating 115 118 113 111 — conditions biaxialtemperature (° C.) stretching Stretching 123 121 122 121 — temperature(° C.) Thermal fixation 50 50 50 50 — temperature (° C.) Magnification(times) 8.2 6.5 6 7 — (1.3 × (1.87 × (1.3 × (1.3 × — 1.8 × 3.5) 1.87 ×1.87) 1.8 × 2.5) 1.8 × 2.99) TDO Preheating 114 114 120 114 —temperature (° C.) Stretching 113 114 120 113 — temperature (° C.)Thermal fixation 85 85 85 105 — temperature (° C.) Magnification (times)8.3 7.7 6.8 8.5 — Maximal deformation rate 2 1 1 3 — deviation(%/second) Simultaneous Preheating — — — — 117 biaxial temperature (°C.) stretching Stretching — — — — 117 temperature (° C.) Thermalfixation — — — — 100 temperature (° C.) Magnification (times) — — — — 5× 5 Heat Temperature (° C.) 132 132 132 131 127 treatment Magnification(times) 1.7→1.6 1.55→1.5 1.5→1.4 1.16→1.10 1.4→1.2 Compar- Compar-Compar- Compar- ative ative ative ative Example 2 Example 3 Example 4Example 5 Resin Ultra high Content proportion (%)  40 2 0 30 compositionpolyethylene (Mw (×10³), f (≥1M)) 2700, 46% 2700, 46% 2700, 46% 2700,46% High density Content proportion (%)  60 98 100 70 polyethylene (Mw(×10³), f (≥1M))  260, 5.0%  260, 5.0%  260, 5.0%  260, 5.0% Solidcontent  23 40 30 30 concentration (%) Stretching Stepwise MDOPreheating — 115 110 117 conditions biaxial temperature (° C.)stretching Stretching — 123 108 117 temperature (° C.) Thermal fixation— 50 80 50 temperature (° C.) Magnification (times) — 8.2 7.5 8 — (1.3 ×— — 1.8 × 3.5) TDO Preheating — 114 124 117 temperature (° C.)Stretching — 113 120 117 temperature (° C.) Thermal fixation — 85 116 85temperature (° C.) Magnification (times) — 8.3 8.4→7 8 Maximaldeformation rate — — — 1 deviation (%/second) Simultaneous Preheating115 — — — biaxial temperature (° C.) stretching Stretching 115 — — —temperature (° C.) Thermal fixation 100 — — — temperature (° C.)Magnification (times) 5 × 5 — — — Heat Temperature (° C.) 127 132 129125 treatment Magnification (times)    1.4 1.7→1.6 1.45→1.2 1

TABLE 2 Comparative Comparative Comparative Comparative ComparativeExample 6 Example 7 Example 8 Example 9 Example 10 Resin Ultra highContent proportion (%) 30 20 30 30 30 composition polyethylene (Mw(×10³), f (≥1M)) 2700, 46% 2700, 46% 2700, 46% 2700, 46% 2700, 46% Highdensity Content proportion (%) 70 80 70 70 70 polyethylene (Mw (×10³), f(≥1M))  260, 5.0%  260, 5.0%  260, 5.0%  260, 5.0%  260, 5.0% Solidcontent 30 30 30 30 30 concentration (%) Stretching Stepwise MDOPreheating 80 130 115 115 116 conditions biaxial temperature (° C.)stretching Stretching 85 135 123 123 116 temperature (° C.) Thermalfixation 50 50 50 50 50 temperature (° C.) Magnification (times)   8.26.5 8.2 8.2 6.51 (1.3 × (1.87 × (1.3 × (1.3 × (1.3 × 1.8 × 3.5) 1.87 ×1.87) 1.8 × 3.5) 1.8 × 3.5) 1.8 × 2.78) TDO Preheating — — 85 125 120temperature (° C.) Stretching — — 90 133 120 temperature (° C.) Thermalfixation — — 85 85 85 temperature (° C.) Magnification (times) — — 8.38.3 7.21 Maximal deformation rate — — 2 2 2 deviation (%/second)Simultaneous Preheating — — — — — biaxial temperature (° C.) stretchingStretching — — — — — temperature (° C.) Thermal fixation — — — — —temperature (° C.) Magnification (times) — — — — — Heat Temperature (°C.) — — — — 132.5 treatment Magnification (times) — — — — 1.61→1.56Comparative Comparative Comparative Comparative Example 11 Example 12Example 13 Example 14 Resin Ultra high Content proportion (%) 20 30 3030 composition polyethylene (Mw (×10³), f (≥1M)) 2700, 46% 2700, 46%2700, 46% 2700, 46% High density Content proportion (%) 80 70 70 70polyethylene (Mw (×10³), f (≥1M))  260, 5.0%  260, 5.0%  260, 5.0%  260,5.0% Solid content 30 30 30 30 concentration (%) Stretching Stepwise MDOPreheating 115 115 115 115 conditions biaxial temperature (° C.)stretching Stretching 123 123 123 123 temperature (° C.) Thermalfixation 50 50 50 50 temperature (° C.) Magnification (times) 4.9 8.28.2 9 (1.3 × (1.3 × (1.3 × (1.3 × 1.5 × 2.5) 1.8 × 3.5) 1.8 × 3.5) 1.8 ×3.85) TDO Preheating 114 114 114 111 temperature (° C.) Stretching 113113 113 110 temperature (° C.) Thermal fixation 85 85 85 85 temperature(° C.) Magnification (times) 8.3 4.9 11 6.6 Maximal deformation rate 2 22 2 deviation (%/second) Simultaneous Preheating — — — — biaxialtemperature (° C.) stretching Stretching — — — — temperature (° C.)Thermal fixation — — — — temperature (° C.) Magnification (times) — — —— Heat Temperature (° C.) 132 — — 132 treatment Magnification (times)1.7→1.6 — — 1.34→1.27

TABLE 3 Compar- Compar- Compar- Compar- Compar- ative ative ative ativeative Exam- Exam- Exam- Exam- Exam- Exam- Exam- Exam- Exam- ple 1 ple 2ple 3 ple 4 ple 1 ple 2 ple 3 ple 4 ple 5 Properties Membrane 12 9 10.911.9 11.1 11.6 9 12 10 before thickness (μm) pressur- Air resistance(sec/100 cm³) 165 155 130 240 130 120 140 130 150 ization Porosity (%)43.6 39.9 44.2 38.2 43.7 45.8 41.2 51.5 47.1 test Pin puncture strength(mN) 5394 3727 4070 4913 3138 3432 3334 5394 4462 Pin puncture strength5394 5001 4511 4952 3432 3530 4511 5394 5394 corresponding to 12 μm (mN)Heat shrinkage rate at 2.8/4.8 3.9/3.8 3.4/2.9 3.0/3.0 6.4/1.2 5.3/5.12.2/3.5 1.4/2.4 9.5/9.8 105° C. (%) MD/TD Tensile rupture strength256/219 252/229 211/178 224/189 127/135 128/164 159/222 207/257 208/251(MPa) MD/TD Tensile rupture elongation 85/85 110/95  120/130 115/135160/145 180/125 130/80 90/60 90/80 (%) MD/TD Average tensile rupture 85102 125 125 152 150 100 74 85 elongation (%) Maximal pore size (nm) 4955 53.5 44.5 50 50 80.5 85 45 Average pore size (nm) 34.5 39 30 27.5 3233 56.5 57.5 30 Impedance 100% 100% 100% 100% 100% 100% 100% 100% 100%SDT (° C.) 139.8 138.8 139.2 138.2 137.2 137 138 142 139.2 MDT (° C.)150.6 149.6 151.2 150.6 150.6 149.6 149 148 150.1 MDT − SDT (° C.) 10.810.8 12 12.4 13.4 12.6 11 6 10.9 Submicron region ^(a)) L L L L 3D 3D LL — Micron region ^(a)) 3D 3D 3D 3D 3D 3D L L — After Membrane thickness(μm) 11 8.8 9.8 10.9 10.2 10.3 8.5 10.3 — pressur- Membrane thicknessvariation  91%  98%  90%  96%  92%  89%  94%  86% — ization Airresistance (sec/100 cm³) 225 225 190 300 195 185 220 215 — at 60° C. Airresistance variation 137% 145% 146% 125% 150% 154% 157% 165% — AfterMembrane thickness (μm) 10.8 8.3 9.6 10.3 9.6 10.1 8.1 10.1 — pressur-Membrane thickness variation  89%  92%  88%  90%  86%  87%  90%  84% —ization Air resistance (sec/100 cm³) 270 275 245 360 280 265 295 310 —at 80° C. Air resistance variation 164% 177% 188% 150% 215% 221% 211%238% — Air resistance variation 120% 122% 129% 120% 144% 143% 134% 144%— ratio of the ratio after pressurization at 60° C. Impedance 103% 102%108% 109% 125% 112% 122% 133% — Impedance retention test ⊚ ◯ ⊚ ◯ ⊚ ⊚ ◯ X— results ^(b)) ^(a)) (L: Ladder-like structure, 3D: three-dimensionalnetwork structure), ^(b)) (⊚: 60 seconds or more, ◯: 45 seconds or more,Δ30 seconds or more, X: less than 30 seconds, —: not reached)

TABLE 4 Compar- Compar- Compar- Compar- Compar- Compar- Compar- Compar-Compar- ative ative ative ative ative ative ative ative ative Exam-Exam- Exam- Exam- Exam- Exam- Exam- Exam- Exam- ple 6 ple 7 ple 8 ple 9ple 10 ple 11 ple 12 ple 13 ple 14 Properties Membrane — — — — 12.1 — —— 13 before thickness (μm) pressur- Air resistance (sec/100 cm³) — — — —120 — — — 240 ization Porosity (%) — — — — 46.6 — — — 38 test Pinpuncture strength (mN) — — — — 4903 — — — 4119 Pin puncture strength — —— — 4805 — — — 3825 corresponding to 12 μm (mN) Heat shrinkage rate at —— — — 2.5/5.8 — — — 2.6/3.0 105° C. (%) MD/TD Tensile rupture strength —— — — 260/178 — — — 213/150 (kPa) MD/TD Tensile rupture elongation — — —— 100/110 — — —  75/140 (%) MD/TD Averaqe tensile rupture — — — — 105 —— — 100 elongation (%) Maximal pore size (nm) — — — — 53 — — — 55Average pore size (nm) — — — — 38 — — — 36 Impedance — — — — 100% — — —100% SDT (° C.) — — — — 142.5 — — — 138.1 MDT (° C.) — — — — 150.4 — — —150 MDT − SDT (° C.) — — — — 7.9 — — — 11.9 Submicron region ^(a)) — — —— L — — — L Micron region ^(a)) — — — — L — — — L After Membranethickness (μm) — — — — — — — — 12.4 pressur- Membrane thicknessvariation — — — — — — — — 0.95 ization Air resistance (sec/100 cm³) — —— — — — — — 370 at 60° C. Air resistance variation — — — — — — — — 154%After Membrane thickness (μm) — — — — — — — — 11.8 pressur- Membranethickness variation — — — — — — — — 0.91 ization Air resistance (sec/100cm³) — — — — — — — — 500 at 80° C. Air resistance variation — — — — — —— — 208% Air resistance variation ratio — — — — — — — — 135% of theratio after pressurization at 60° C. Impedance — — — — — — — — 120%Impedance retention test — — — — — — — — ⊚ results ^(b)) ^(a)) (L:Ladder-like structure, 3D: three-dimensional network structure), ^(b))(⊚: 60 seconds or more, ◯: 45 seconds or more, Δ30 seconds or more, X:less than 30 seconds, —: not reached)

The invention claimed is:
 1. A microporous membrane, wherein average membrane thickness is 15 μm or less, air resistance is 400 seconds or less, pin puncture strength corresponding to a membrane thickness of 12 μm is 4000 mN or more, and air resistance variation (60° C.) after a pressurization treatment under conditions of 60° C., 4 MPa, and 10 minutes is 148% or less: wherein Air resistance variation (60° C.)=air resistance after the pressurization treatment at 60° C./air resistance prior to the pressurization treatment×100.
 2. The microporous membrane according to claim 1, wherein said air resistance variation is 145% or less.
 3. The microporous membrane according to claim 1, wherein the air resistance variation (80° C.) after a pressurization treatment under conditions of 80° C., 4 MPa, and 10 minutes is 200% or less: wherein Air resistance variation (80° C.)=air resistance after the pressurization treatment at 80° C./air resistance prior to the pressurization treatment×100.
 4. The microporous membrane according to claim 3, wherein said air resistance variation (80° C.) is 190% or less.
 5. The microporous membrane according to claim 1, wherein pin puncture strength corresponding to the membrane thickness of 12 μm is 4000 mN or more, heat shrinkage rate after an exposure at 105° C. for 8 hours is 5% or less, and average tensile rupture elongation is 130% or less.
 6. The microporous membrane according to claim 1, wherein at least one of the following is satisfied: the shutdown temperature is 140° C. or less, and the temperature difference between the shutdown temperature and the meltdown temperature obtained by a temperature-increasing air permeability method is 10° C. or more.
 7. The microporous membrane according to claim 1, wherein average pore size is 0.1 μm or less.
 8. The microporous membrane according to claim 1, comprising 2% or more of an ultra high molecular weight polyethylene component having a weight average molecular weight of 1,000,000 or more, or 5% or more of a molecule component having a weight average molecular weight of 1,000,000 or more.
 9. The microporous membrane according to claim 1, wherein formation of a hybrid structure of a ladder structure in a submicron region and a three-dimensional network structure in a micron region is observed on at least one surface of the microporous membrane.
 10. A lithium ion secondary battery, wherein the microporous membrane according to claim 1 is used. 